1. Field of the Invention
The present invention relates generally to direct bonding of copper on ceramic substrates and, more particularly, to the direct bonding of copper on aluminum nitride.
2. Background Information
It has been determined that aluminum nitride (AlN), when sufficiently pure, has a thermal conductivity which is near that of beryllia (BeO). This characteristic makes aluminum nitride a most desirable dielectric for use as a substrate where thermal resistance of the package is a limiting factor. Further, aluminum nitride has a temperature coefficient of expansion (TCE) close to that of a silicon (Si) chip and possesses a thermal conductivity, as previously indicated, approximately ten times that of alumina.
Before a ceramic substrate may be used in the assembling of power devices, it must first be provided with a highly conductive metallization. Normally, copper is chosen for this metallization, in the thickness range of 0.010-0.020 inches. The use of copper metallization in this fashion ensures low electric resistive losses. Direct bonding of metal to ceramic substrates has been known for some time. A significant number of patents have been issued on the subject of direct bond copper (DBC) on ceramic substrates, most notably on alumina or beryllia. See, for example, D. A. Cusano et al. U.S. Pat. No. 3,994,430, "DIRECT BONDING OF METALS TO CERAMICS AND METALS", issued Nov. 30, 1976 and G. L. Babcock et al. U.S. Pat. No. 3,766,634, "METHOD OF DIRECT BONDING METALS TO NON-METALLIC SUBSTRATES", issued Oct. 23, 1973, both of which are hereby incorporated by reference. DBC on aluminum nitride has been used by the Toshiba Corporation in its line of power modules. More recently, a paper by Kuromitsu et al. of the Mitsubishi Metal Corporation, in the publication of the International Society for Hybrid Microelectronics (ISHM)) 1989, entitled "Surface Treatment of AlN Substrate", discusses a metal bond-enhancing surface treatment for AlN substrate. Iwase et al. of the Toshiba Research and Development Center in the publication of the IEEE Components, Hybrids, and Manufacturing Technology, Vol. CHMT-8, No. 2, June 1985, pp 253-258 present a paper entitled "Thick Film and Direct Bond Copper Forming Technologies for Aluminum Nitride Substrate" which relates a certain degree of success in applying DBC technique to a lightly doped (3% yttria) AlN sintered substrate. AlN is one of the most promising candidates as a highly thermally conductive substrate for semiconductor devices. Kuromitsu et al. and Iwase et al. propose a surface treatment for an AlN substrate that would make it suitable to receive most thick film materials that are now used on Al.sub.2 O.sub.3 (alumina) substrates. AlN, because of certain superior properties such as high thermal conductivity, low permittivity and high mechanical strength, is indeed a promising candidate as a replacement for alumina, and even beryllia (which is a relatively toxic substance) substrates, if the poor adhesion of films to AlN substrates can be overcome. The instant invention succeeds in overcoming this widely known limitation of metal bonding to an aluminum nitride substrate by providing a method of controlled conversion of AlN to Al.sub.2 O.sub.3 to obtain a thin stratum of the oxide on the nitride.
Before discussing the Kuromitsu et al. and Iwase et al. techniques, it is necessary to address recent developments in the preparation of AlN substrates. A cheaper version of AlN of somewhat higher thermal resistivity, but still several times lower than that of alumina, has recently been achieved. The newer AlN is sintered, and is correspondingly doped with a large fraction (up to 10%) of yttria which acts as a sintering aid in the fabrication of the substrate. Although resulting in a strong substrate which possesses most of the aforementioned enhanced properties, it has been established that the direct bonding process, most notably DBC, is unsatisfactory under certain conditions; moreover, the earlier published Toshiba recipe, although scrupulously followed, does not result in adherent copper films.
An adherent copper (or metal) film, as the term is used herein, means one which has high peel strength when applied on the desired or preferred dielectric substrate. Further, the entire ensemble should possess the high thermal conductivity necessary for dissipation of heat in electronic power modules. Optimally, direct bonded copper (DBC) on beryllia adequately fulfills these criteria; however, unencapsulated beryllia is sometimes considered too toxic for consumer applications. The next favored structure is DBC on AlN, a combination which results in good thermal conductivity but very poor peel strength, as pointed out in the Kuromitsu et al. paper. The least desirable of all the metallized structures is metallization (typically Cu) on Al.sub.2 O.sub.3. This latter combination exhibits high peel strength, but possesses poor thermal conductivity with DBC. In fact, the thermal conductivity tradeoff (for the high peel strength) is a loss so great that, in situations requiring good thermal conductivity, use of one of the first mentioned structures is compelling. Since the use of beryllia is preferably avoided in certain applications because of the toxicity question, the DBC as taught herein constitutes a significant improvement over the Toshiba and Mitsubishi AlN substrates relative to the adherence of thin films of copper thereto. More specifically, the pre-treatment of the new yttria-doped AlN substrate that is set forth herein results in optimum adhesion when undergoing the DBC process set forth in the aforementioned Babcock et al. patent, for example.
The surface treatment of AlN substrates by Kuromitsu et al. and Iwase et al. comprises, generally, formation of an Al.sub.2 O.sub.3 layer on the surface of AlN substrate. Although the Kuromitsu et al. technique also includes mixing alumina with certain glasses, they nevertheless report obtaining sufficient bonding strengths, and, by use of SiO.sub.2, they achieve excellent bond strength. It was observed by Kuromitsu et al. that, when the Al.sub.2 O.sub.3 layer is thin, a thicker SiO.sub.2 layer is required to obtain this excellent bond strength. They conclude with the statement that the desirable thickness of Al.sub.2 O.sub.3 is thought to be "5 to 8 .mu.m" in order to retain the excellent thermal conductivity of the AlN substrate. Unfortunately, reliance on the various glasses used in the Kuromitsu et al. process is not always desirable and, even worse, appears to result in a loss in thermal conductivity of the AlN substrate and an increase in outgassing, caused initially by dissolution of the melt glass into the AlN substrate. Thus, by using the published Kuromitsu et al. process, loss of ultimate peel strength is encountered, induced by increased outgassing.
As hereinafter discussed, the instant process for effecting a DBC on aluminum nitride includes a pretreatment of the aluminum nitride that is superior to the Toshiba and Mitsubishi teachings and overcomes the previously mentioned limitations of the technology. The doped AlN has strength superior to intrinsic AlN (substrate) because the yttria enhances sintering of the AlN (powder) used to fabricate the substrate. As a consequence, glasses are not required for good adhesion strength and a thinner Al.sub.2 O.sub.3 layer (not susceptible to crazing or cracking) permits easier eutectic bonding and provides higher thermal conductivity.